Non-reciprocal circuit devices such as isolators, etc. are widely used in mobile communications apparatuses utilizing frequency bands from several hundreds MHz to ten-odd GHz, such as base stations for personal handy phones PHS, cell phones, etc. The isolator is disposed, for instance, between a power amplifier and an antenna in a transmission path in a mobile communications apparatus, to prevent unnecessary signals from flowing back to the power amplifier and stabilize impedance on the load side of the power amplifier. Accordingly, the isolator is required to have excellent insertion loss characteristics, reflection loss characteristics and isolation characteristics.
FIG. 8 shows a three-pair-terminal isolator as one example of such isolators. This isolator comprises a microwave ferrite 38, three electrically-insulated central conductors 31, 32, 33 crossing each other at an angle of 120° on a main surface of the microwave ferrite 38, matching capacitors C1-C3 each connected to one end of each central conductor 31, 32, 33, and a termination resistor Rt connected to any one port (for instance, P3) of the central conductors 31, 32, 33. The other end of each central conductors 31, 32, 33 is connected to the ground. A DC magnetic field Hdc is axially applied from a permanent magnet (not shown) to the ferrite 38. In this isolator, a high-frequency input from a signal port P1 is transmitted to a port P2, while reflected waves from the port P2 are absorbed by the termination resistor Rt without being transmitted to the port P1. Thus, unnecessary reflected waves are prevented from inversely entering to a power amplifier, etc.
Recently proposed is an isolator constituted by a different equivalent circuit from that of such three-pair-terminal isolator and having excellent insertion loss characteristics and reflection loss characteristics (JP2004-88743A). This isolator having two central conductors is called “two-pair-terminal isolator.” FIG. 9 shows an equivalent circuit of the two-pair-terminal isolator, and FIG. 10 is an exploded perspective view showing its parts. This two-pair-terminal isolator comprises a first inductance element L1 formed by a first central conductor 21 disposed between a first input/output port P1 and a second input/output port P2, a second inductance element L2 formed by a second central conductor 22 disposed between the second input/output port P2 and the ground such that it crosses the first central conductor 21 with electric insulation, a first capacitance element C1 disposed between the first input/output port P1 and the second input/output port P2 for constituting a first parallel resonance circuit with the first inductance element L1, a resistance element R, and a second capacitance element C2 disposed between the second input/output port P2 and the ground for constituting a second parallel resonance circuit with the second inductance element L2.
When a high-frequency signal is conveyed from the first input/output port P1 to the second input/output port P2, the first parallel resonance circuit between the first input/output port P1 and the second input/output port P2 is not resonated, while the second parallel resonance circuit is resonated, resulting in small transmission loss (excellent insertion loss characteristics). Current inversely flowing from the second input/output port P2 to the first input/output port P1 is absorbed by the resistance element R between the first input/output port P1 and the second input/output port P2.
As shown in FIG. 10, the two-pair-terminal isolator 1 comprises cases (upper case 4 and lower case 8) made of ferromagnetic metals such as soft iron, etc. to constitute a magnetic circuit, a permanent magnet 9, a central conductor assembly 30 comprising a microwave ferrite 20 and central conductors 21, 22, and a multilayer substrate 50, onto which the central conductor assembly 30 is mounted. The central conductor assembly 30 comprises a disc-shaped microwave ferrite 20, and first and second central conductors 21, 22 disposed on an upper surface of the microwave ferrite 20 such that they cross each other via an insulating layer (not shown). Each of the first and second central conductors 21, 22 is constituted by two lines, both end portions of each line extending along a lower surface of the microwave ferrite 20 with mutual separation.
The multilayer substrate 50 comprises a first capacitance element C1 constituting the first parallel resonance circuit, a second capacitance element C2 constituting the second parallel resonance circuit, and a resistance element R. FIG. 11 is an exploded perspective view showing each part of the multilayer substrate 50. The multilayer substrate 50 comprises electrodes 51-54 connected to the end portions of the central conductors 21, 22, a dielectric sheet 41 provided with capacitor electrodes 55, 56 and a resistor 27 on the rear surface, a dielectric sheet 42 provided with a capacitor electrode 57 on the rear surface, a dielectric sheet 43 provided with a ground electrode 58 on the rear surface, dielectric sheets 44, 45 provided with an external input electrode 14, an external output electrode 15 and external ground electrodes 16, etc. The capacitor electrodes 55, 57 constitute the first capacitance element C1, and the capacitor electrodes 56, 57 constitute the second capacitance element C2. Black circles show via-holes in the figure.
One end portion of the first central conductor 21 is connected to the external input electrode 14 via the electrode 51. The other end portion of the first central conductor 21 is connected to the external output electrode 15 via the electrode 54. One end portion of the second central conductor 22 is connected to the external output electrode 15 via the electrode 53. The other end portion of the second central conductor 22 is connected to the external ground electrode 16 via the electrode 52.
In the two-pair-terminal isolator, a resonance frequency (hereinafter referred to as “peak frequency”) for providing the maximum isolation is determined by adjusting the first inductance element L1 and the first capacitance C1 formed by the first central conductor 21, and a peak frequency for providing the minimum insertion loss is determined by adjusting the second inductance element L2 and the second capacitance C2 formed by the second central conductor 22. Thus, the electric characteristics of the two-pair-terminal isolator are determined by adjusting the first and second inductance elements L1, L2 and the first and second capacitances C1, C2 depending on the frequency of a communications system used in a communications apparatus. Accordingly, to obtain a two-pair-terminal isolator with excellent electric characteristics, it is important to form the first and second inductance elements L1, L2 and the first and second capacitance elements C1, C2 with high accuracy and little variations.
However, because the inductance and capacitance of such parts vary by their various factors, it is almost difficult to get a constant peak frequency, resulting in many two-pair-terminal isolators failing to have the desired electric characteristics.
The variations of electric characteristics caused by the first and second inductance elements L1, L2 can be reduced by adjusting the magnet force of the permanent magnet by a magnetizing coil, because their inductances are determined by the widths of the central conductors and their gaps, the magnetic characteristics and size of the microwave ferrite, and the DC magnetic field generated from the permanent magnet. With respect to the variations of electric characteristics caused by the first and second capacitance elements C1, C2 formed in the multilayer substrate 50, however, the variations of their capacitances can be reduced only to about ±4% even though various conditions such as the thickness of electrodes, dielectric sheets, etc. are controlled at high accuracy, because their capacitances are determined by the dielectric characteristics of dielectric sheets, the areas and gaps of capacitor electrodes, etc.
In a two-pair-terminal isolator for an 800-MHz band, for instance, its peak frequency shifts by several MHz when the capacitances of the first and second capacitance elements C1, C2 vary 1% from the desired level. When the variations exceed ±3%, the two-pair-terminal isolator faills to meet its standards. Accordingly, the capacitance variations should be within ±3%, preferably within ±2%, of the desired capacitance.
As described above, because it is difficult to suppress capacitance variations by adjusting the thickness of electrodes, dielectric sheets, etc., electrode patterns formed in the multilayer substrate have conventionally been trimmed by a laser. However, the capacitance adjustment of the first and second capacitance elements C1, C2 by trimming causes the breakage, cracking, etc. of the multilayer substrate, resulting in an extremely reduced production yield.